1. Field of the Invention
The present invention relates to an image projection device employing a pixel shift technique, and more particularly, relates to an optical system, an optical engine, an optical unit, and a projection unit of the image projection device.
2. Description of the Related Art
In a display device having a light valve element formed from plural pixels corresponding to different colors and arranged in a matrix manner, such as a projector or a head mounted display, the number of the pixels of the light valve element is increasing every year. Specifically, the light valve element may be a spatial optical modulation device, such as a liquid crystal light valve, or an LCoS (liquid crystal on Silicon), or a DMD (Digital Mirror Device). Along with the increase of the pixel number, the pixel size becomes smaller and smaller, and a pixel driving mechanism becomes fine and more complicated, causing an increases of the cost of the display device. In addition, when the pixel size becomes small, the ratio of effective pixel area to total area may decrease, and this may cause declination of light utilization. On the other hand, if the pixel number is increased without decreasing the pixel size, the size of a display element increases, the size of the optical system for the display element also increases, and this also causes an increase of the cost.
There exists a technique able to increase the effective pixel number without increasing the number of the pixels of the display element, which is known as “pixel shift technique”, and is also referred to as a “wobbling technique”. Specifically, the pixel shift technique can shift the pixels by a distance less than a pixel size on a display plane for a short time, so that it displays un-shifted pixels at normal positions and shifted pixels at the shifted positions on the display plane alternatively in a time-division manner, or displays the pixels shifted by different distances thus at different positions on the display plane in a time-division manner. Alternatively, it displays the un-shifted pixels and the shifted pixels at the same time with the un-shifted pixels and the shifted pixels being overlapped, or displays the shifted pixels at the same time with the shifted pixels being overlapped.
When displaying pixels with their positions being changed in the time-division manner, an image at a first position is viewed due to an after image effect on human eyesight, so that while the image at the first position is being viewed, an image at a second position is displayed, and it appears as if the number of pixels has been doubled.
Alternatively, plural display elements may be used, and pixel positions of the display elements may be slightly shifted relative to each other, allowing images of the display elements to be overlapped with each other. For example, the distance by which the pixels are shifted may be a half or a quarter of the pixel size.
The liquid crystal may be used for shifting the display positions of the pixels. Specifically, an optical axis of the light passing through the liquid crystal may be deflected (it is equivalent to “shifted” in a restricted meaning), and the deflected light is projected onto a projection plane to display an image corresponding to pixels of the display element.
Utilizing the birefringence characteristics of the liquid crystal, when the alignment angle of the liquid crystal is inclined relative to the optical axis, and the principal axis of the liquid crystal molecules is inclined relative to the optical axis, an extraordinary light ray component is subjected to the birefringence effect. In addition, the alignment angle of the liquid crystal molecules can be switched by the voltage applied on the liquid crystal layer. Therefore, by using an element including liquid crystal and a unit for applying a voltage on the liquid crystal, it is possible to switch a shift operation of the optical axis of the light passing through the liquid crystal. This pixel shift technique is a well-known technique.
When displaying images using the pixel shift technique, while the image at the first position is being viewed due to the after image effect, the pixel shift is performed and an image is displayed at the second position. This is quite effective to increase the resolution of the displayed image. In order to support display of moving pictures, generally, the frequency of changing the image frame is 60 Hz or higher; thus the pixel shift should be performed at a frequency higher than the frame display frequency. Therefore, it is necessary to change the alignment of the liquid crystal quickly. It is known that a ferroelectric liquid crystal has a fast response for voltage application. Further, it is known that a perpendicularly-aligned ferroelectric liquid crystal, the principal axis of the liquid crystal being orientated along the thickness direction of the liquid crystal layer, is a material preferable for high speed pixel shift operations.
A pixel shift element utilizing a liquid crystal alignment control technique using the birefringence characteristics of the perpendicularly-aligned ferroelectric liquid crystal and a voltage application unit is described in T. Tokita et al., “FLC Resolution-Enhancing Device for Projection Display”, Society for Information Display 2002 International Symposium Digest of Technical Papers, USA, May 2002, Volume XXXIII, Number I, pp. 638-639 (referred to as “reference 1” hereinafter). In this pixel shift element, dielectric thin films are provided on two sides of a liquid crystal layer, and ITO thin film electrode layers are arranged outside the dielectric thin films, and the thus obtained structure is sandwiched by cover glass; the optical axis is defined to be a Z axis, a plane perpendicular to the optical axis is defined to be an X-Y plane; by switching ON/OFF voltage application, a tilt angle θ, which is defined to be the angle between the principal axis of the liquid crystal and the optical axis, is switched along the ±Y direction in the YZ plane to shift an extraordinary light ray component of the light incident into the liquid crystal layer parallel to the ±Y direction according to the direction of the liquid crystal. By utilizing this light path shift technique, it is possible to shift the pixels in four directions, namely, in the ±X and ±Y directions.
Typical pixel shift operations include a step of displaying a pixel image at the first position, a step of shifting the pixel image to the second position, a step of displaying a pixel image at the second position, and a step of shifting the pixel image to the first position. Since each pixel is shifted and displayed at two positions, the number of displayed pixels is apparently doubled, or in other words, the number of image frames formed from the pixels is apparently doubled.
Further, the pixels can be shifted in both the horizontal direction and the vertical direction, thereby, apparently increasing the number of displayed pixels by four times. In this case, a mechanism able to shift the pixels in both the horizontal direction and the vertical direction is required. For this purpose, for example, a pixel shift element for shifting the alignment direction such as the perpendicularly-aligned ferroelectric liquid crystal in the ±X direction, and a pixel shift element for shifting the alignment direction of liquid crystal in the ±Y direction can be used together. In addition, if the horizontal direction and vertical direction of light valve elements are in agreement with the X direction and the Y direction, the pixels can be shifted in four directions, namely, in the ±X and ±Y directions. As a result, the number of displayed image frames is apparently increased by four times, and the amount of displayed image data is also increased by four times.
As another issue in image display using the pixel shift technique, if the pixel image being shifted is displayed, pixels appear to be connected to each other, and especially, when the image data before and after the pixel shift operation are quite different, because of connection of neighboring pixel images, pixel images are not well separately and this degrades the resolution of the displayed image.
For example, Japanese Laid-Open Patent Application No. 9-15548 (referred to as “reference 2” hereinafter) discloses a technique of preventing display of pixels being shifted. In the strict sense, this reference primarily focuses on a liquid crystal panel having pixels of a Delta arrangement, but prevention of displaying pixels being shifted is also mentioned.
As another issue in image display using the pixel shift technique, it is well known that an image formed from pixel images of a display element is read out from a frame memory frame by frame, and is updated. Generally, in the frame update of the display element, if pixel images of all pixels are updated at the same time, a very high operating speed is required. Due to this, the frame update is usually performed line-sequentially (line-sequential scheme). Each scanning line includes pixels arranged along a line.
FIG. 11 is a diagram illustrating a timing of updating scanning lines and a timing of pixel shifting in the related art. In FIG. 11, a symbol “T” indicates time.
As shown in FIG. 11, in each frame, the pixel shift operations are performed twice or more. Here, a frame displayed after the pixel shift is referred to as a “sub-frame”. The speed of updating the sub-frame should be sufficiently fast so that the time required for updating the sub-frame is shorter than the time required for displaying the sub-frame. Due to this, it is more difficult to update all pixels of one frame at the same time, and it is more preferably to update one frame line-sequentially.
However, another problem arises in the relation between the timing of pixel shifting and the timing of updating the sub-frame image. During the period of image frame updating, if the pixel shift is completed during the updating period while controlling the display element such that the display grade is at zero level, the image of the pixels being shifted is not displayed. Nevertheless, the timing of the image frame updating is delayed line-sequentially.
In FIG. 11, the abscissa represents a time axis, and the ordinate represents the direction of scanning line series; T1 represents the start time of updating the first scanning line, and T2 represents the end time of updating the first scanning line; T3 represents the start time of updating the last scanning line, and T4 represents the end time of updating the last scanning line; T5 represents the start time of the pixel shift, and T6 represents the end time of the pixel shift.
As shown in FIG. 11, time T3 is later than T1, and T4 is later than T2. This is the above-mentioned delay of the timing of the image frame updating.
Further, as shown in FIG. 11, in the period from the start time T3 of updating the last scanning line to the end time T2 of updating the first scanning line, the pixel shift is not finished, hence, in the hatched regions in FIG. 11, the pixel image being shifted ends up being displayed. In addition, in the hatched regions, the pixel shift is started while the previous frame is still being displayed, and similarly, the pixel image being shifted ends up being displayed.
In order that the pixel image being shifted is not displayed, it is necessary to set the time required for shifting the pixels (that is, T6−T5) to be shorter than the time required for updating the whole display element (that is, T4−T1). Alternatively, it is necessary to set an excessive standby time period so that even when frame updating is finished, the next sub frame is not displayed until the pixel shift is finished. However, in this case, time not used for displaying images increases, and light utilization declines; as a result, the displayed image is not bright enough. The above reference 2 does not mention these problems.
Japanese Laid-Open Patent Application No. 6-324320 (referred to as “reference 3” hereinafter) discloses a method of shifting the pixels in synchronization with vertical scanning of the scanning lines. As described in reference 3, in this method, employing the polarization effect, pixels are shifted and displayed only in a scanning line region where the frame updating has been performed. In the meantime, in the scanning line region in the state of the preceding frame, since the polarization effect does not occur, display positions of the pixels therein are not shifted. However, the above-described problems occurring in the line-sequential operations during the frame updating period are not mentioned.
FIG. 12 is a diagram illustrating an ideal state in which pixel shift is performed in the period of updating the scanning lines while following delay of the updating timing in the related art.
In FIG. 12, similar to FIG. 11, the abscissa represents the time axis, the ordinate represents the direction of scanning line series, and the symbol “T” represents time.
In the line-sequential frame updating operations, in order to maintain the brightness of the displayed image, as shown in FIG. 12, preferably, pixels are shifted line-sequentially in a region corresponding to pixels on scanning lines which have been updated.
In FIG. 12, T5 represents the start time of pixel shift of the first scanning line, and T6 represents the end time of the pixel shift of the first scanning line; T7 represents the start time of pixel shift of the last scanning line, and T8 represents the end time of the pixel shift of the last scanning line. In FIG. 12, the start time and the end time of the pixel shift are delayed line-sequentially. Thereby, it is possible to shorten or diminish the time difference between the time of image updating and the time of pixel shift; hence, it is possible to improve the brightness of the screen. In FIG. 12, if T1=T5, T2=T6, T3=T7, and T4=T8, the time difference becomes zero, and the time not used for display does not exist, allowing the most sufficient image display.
However, the sequential pixel shift in order of scanning lines cannot be performed by a pixel shift optical system in the related art.
FIG. 13 is a schematic view of an optical system for pixel shift in the related art.
The optical system shown in FIG. 13 includes a light valve 1, a polarized-beam splitter 2, a color combination prism 3, a pixel shift element 4, and a magnification projection system 5. A light beam from the light valve 1 transmits through the polarized-beam splitter 2 and the color combination prism 3, and enters into the pixel shift element 4; the pixel shift element 4 shifts the path of the light beam, and the magnification projection system 5 projects the pixel-shifted image.
It should be noted that generally the light valves 1, the polarized-beam splitters 2, and illumination systems respectively corresponding to R, G, B three colors are provided in the above pixel shift optical system, but in FIG. 13, only one light valve 1 and one polarized-beam splitter 2 are illustrated for descriptive purposes.
In the above pixel shift optical system, as shown in FIG. 13, a light beam output from one pixel of the light valve 1 is spread and is incident on the pixel shift element 4. Thus, light beams from pixels of the light valve 1 on different scanning lines are partially overlapped with each other before entering into the pixel shift element 4, and it is difficult to spatially separate the light beams corresponding to different pixels. For this reason, it is difficult to perform the sequential pixel shift in order of scanning lines corresponding to the pixels of light valve 1.
FIG. 14 is a schematic view of an image display device using a color wheel. In FIG. 14, the same reference numbers are assigned to the same elements as those illustrated in FIG. 13.
The image display device in FIG. 14 includes a lamp 6 acting as a light source, a color wheel 7, and a fly-eye lens array 8.
A light beam from the lamp 6 is converted into a Red (R), Green (G), or Blue (B) monochromatic light beam, is homogenized in light intensity and polarized to have a polarization plane along a specified direction by the fly-eye lens array 8, and incident into the light valve 1. In the light valve 1, the polarization direction of the light beam modulated according to image data is changed by 90 degrees, passes through a polarization reflection surface and enters into the pixel shift element 4. The subsequent operations are the same as those described with reference to FIG. 13.
As shown in FIG. 14, in an optical system using a light valve, and sequentially displaying R, G, B images with a color wheel, the same effect can be obtained by using the pixel shift element 4 as the optical system in FIG. 13, which uses optical systems respectively corresponding to R, G, B three colors (although illustrated only partially).
Similar to the optical system in FIG. 13, as it is difficult to spatially separate the light beams from different pixels corresponding to scanning lines of the light valve 1, it is difficult to perform the sequential pixel shift in order of scanning lines.
Namely, in the image display device employing the pixel shift technique, in the related art, when the perpendicularly-aligned ferroelectric liquid crystal is used as the pixel shift element, because the light valve and the pixel shift element are separated from each other, the timing of the pixel shift cannot be performed in synchronization with the timing of line-sequentially updating the image data, and this causes degradation of image quality. If the updating duration is lengthened in order to avoid this problem, the average image brightness becomes insufficient.